Composite structures typically include structural reinforcing fibers embedded in a resin matrix. Certain areas of a composite layup may include resin-rich pockets which may be described as locations in the composite layup that have a large volume of resin relative to the volume of fibers at the location. Resin-rich pockets may occur between unidirectional fiber tows of a unidirectional ply, at the intersections of fiber tows of woven fabric, at ply steps of a thickness-tapered composite laminate, and at other locations in other types of composite structures.
During the manufacturing of a composite structure, a composite layup may be prepared by applying resin to reinforcing fibers. The temperature of the composite layup may be increased to reduce the viscosity of the resin so that the resin may flow and infuse into the fibers. The composite layup may be held at an elevated temperature for an extended period of time to cure the resin into a solidified or hardened state. After the resin has cured, the composite structure may be cooled to ambient temperature.
In many composite material systems, the resin may have a coefficient of thermal expansion (CTE) that may be different than the CTE of the fibers. For example, epoxy resins may have a CTE that may be an order of magnitude greater than the CTE of carbon fibers. The difference in CTE may result in the resin and fibers contracting by different amounts when the temperature of the composite structure is cooled after curing. The difference in contraction of the resin relative to the fibers may result in thermally-induced stresses in the resin-rich pockets. The thermally-induced stresses may result in undesirable cracking or microcracking in the resin-rich pockets due to the relatively large volume of solidified resin in the resin-rich pockets and the corresponding greater amount of contraction of the resin relative to the fibers in the resin-rich pockets. Microcracking may also occur during the service life of a composite structure due to changes in temperature of the operating environment.
A composite structure may be designed to accommodate a certain amount of microcracking by increasing the structural load-carrying capability of the composite structure. Unfortunately, increasing the load-carrying capability of the composite structure to accommodate microcracking may result in an increase in structural weight which may reduce the performance efficiency of the composite structure. For example, in the case of an aircraft comprised of load-carrying composite structures, an increase in the structural weight of the composite structures may result in a reduction in the range, fuel efficiency, and/or payload capability of the aircraft.
As can be seen, there exists a need in the art for a system and method for preventing or mitigating the occurrence of microcracking in a composite structure during the manufacturing and use of the composite structure.